The work proposed is to determine how steroids interact with cell membranes, whose lipids are undergoing graded degrees of free radical damage. Recent work indicated that naturally occuring membrane sterols, e.g., cholesterol, can modulate free radical lipid reactions within membranes, using in vitro and in vivo model systems. Synthetic corticosteroids appear to have a greater modulating effect than cholesterol, on a mole for mole basis. The present work will employ in vitro (liposomes) and in vivo (CNS injury) models wherein radical reactions are known to occur. Different types of corticosteroids will be interacted with these models, and subsequent analyses made of the physicochemical alterations in the steroid, as well as in the other ampipathic membrane lipids.